This invention relates to a method for manufacturing a plywood using an improved veneer bonding technique.
In prior art techniques of bonding veneers for plywood manufacture, it is well known that poor adhesion such as puncture and deficient bond strength largely depends upon the extent of drying of veneers, that is, the moisture content of veneers.
However, it is impossible at the present state of the art to dry a number of veneers to an equal moisture content, and veneers are dried to a more or less varying extent. Particularly, the recent deterioration of log stock results in a mixed supply of difficult- and easy-to-dry woods. With the increased extent of drying, over-drying results in excessive shrinkage and corrugation of veneers. With the reduced extent of drying, on the other hand, under-drying results in poor adhesion. In connection with veneer drying, a number of problems arise at the site of manufacture.
The only one solution for the present status depends on the development of a practical gluing technique which is completely or substantially independent from variable moisture contents. To essentially eliminate these problems, it is necessary to establish a technique capable of gluing veneers having a high moisture content at low cost.
One technique of gluing veneers having a high moisture content is disclosed in Japanese Patent Application Publication No. 54-3929 titled "Plywood Manufacturing Method". According to the method of this patent publication, a plywood adhesive fluid of well-known formulation is applied to a veneer and dried thereon, and another veneer is placed on the adhesive-applied veneer followed by hot pressing. Although the results described in the patent publication are satisfactory, I found that this technique has the following problems. With a dryer equipped for the purpose of drying the adhesive applied and set to proper drying conditions including temperature and time, the adhesive applied on those veneers having a lower moisture content is over-dried and the adhesive applied on those veneers having a higher moisture content is under-dried, failing to dry the adhesive applied to a desired extent for all veneers. As a result of the varying dryness of the adhesive, the bond strength achieved by hot pressing also varies over a wide range.
In the examples described in the patent publication, drying conditions are changed in accordance with the moisture content of veneers to be handled, and more specifically, the higher the moisture content of veneers, the higher the drying conditions are set. The dryness of the adhesive applied not only depends on the selected adhesive drying conditions, but largely depends on the moisture content of veneers. The varying bond strength is attributable to this fact. Consequently, the dryer for drying the adhesive applied to veneers must be of size and capacity as large as the conventional veneer dryers. Besides, it is well known that there is a significant difference between sapwood and heartwood portions of the same log and between spaced portions of the same veneer. From a standpoint of view of manufacturing plywoods of consistent quality from a variety of starting logs, the above-mentioned shortcomings of the prior art techniques are difficult to overcome in the actual forms of plywood manufacture requiring large-scale production and prevent commercial application of such techniques.
An object of this invention is to eliminate the above-mentioned shortcomings involved in the prior art techniques and to provide a novel and improved technique for firmly bonding veneers, even highly wet veneers if necessary, at low cost independent of the moisture content of veneers so that the technique may be readily applied to commercial manufacture.